A vertical contour correcting circuit extracts a vertical luminance difference signal of a luminance signal and adds the extracted signal to the luminance signal at a certain level, thereby enhancing the contour in the vertical direction. FIG. 1 shows a prior art vertical contour correcting circuit. In this prior art circuit, an NTSC color video signal digitized at a sampling frequency of 4 f.sub.sc (where f.sub.sc is a subcarrier frequency), and its delayed versions delayed by 1 H (one horizontal interval) and 2 H, i.e., 1 H and 2 H delayed video signals, are supplied. The color video signal and 1 H and 2 H delayed video signals are supplied to a vertical BPF (band-pass filter) 11 in the vertical contour correcting circuit. The vertical BPF 11 extracts a level difference for a vertical DC component of the luminance signal, with its center frequency located at f.sub.h /2 (where f.sub.h is a horizontal scan frequency). With the vertical BPF 11, the color video signal and 2 H delayed video signal are added by an adder 111, and the resulting sum is provided to a subtractor 113 via a 1/2 multiplier 112. The subtractor 113 subtracts the output signal of the 1/2 multiplier from the 1 H delayed video signal, and provides the resulting difference to a 1/2 multiplier 114. The output signal of the 1/2 multiplier 114 represents an output signal of the vertical BPF 11. Assuming that the color video signal, 1 H delayed video signal and 2 H delayed video signal are denoted by a, b, and c, respectively, then the output signal of the vertical BPF 11 is (b-(a+c)/2)/2.
Since the output signal of the vertical BPF 11 contains a frequency portion of 3.58 MHz, the 3.58 MHz frequency portion is removed from the output signal of the vertical BPF 11 by a LPF (low-pass filter) 12 having a notch at 3.58 MHz. The LPF 12 has four delay elements 121--124 connected in series from input terminal IN. The delay elements 121-124 delay the signal by 70 nsec (sampling period) as delay time T. In addition, an adder 126 is provided which adds the input signal applied to the input terminal IN with the output signal of the delay element 124, while an adder 126 is connected to the output of the delay element 122. The adder 126 adds the output signal of the adder 125 to the output signal of the delay element 122. That is, assuming that the signal of the input terminal IN, the output signal level of the delay element 122, and the output signal level of the delay element 124 are denoted by A, B, and C, respectively, then the output level of the adder 126 is represented by (B+(A+C)/2)/2, so that only a DC portion is extracted. It should be appreciated that the adders 125 and 126 are capable of scaling by half and outputting the signal level of their resultant sum.
For the output signal of the LPF 12, its signal portion below a predetermined level is cut by a noise slice circuit 13, and then supplied to a clip circuit 14. The clip circuit 14 limits a high-level portion of the output signal of the noise slice circuit 13. The noise slice circuit 13 and clip circuit 14 have an input/output characteristic as shown in FIG. 3.
On the other hand, the color video signal and 1 H and 2 H delayed video signals are supplied to a comb filter 15. The comb filter 15 extracts a carrier color signal contained in the color video signal. The 1 H delayed video signal is supplied to a subtractor 16, where the carrier color signal extracted by the comb filter 15 is subtracted from the 1 H delayed video signal to extract a luminance signal. The resulting luminance signal is supplied to an adder 17, where it is added with the output signal of a noise slice circuit 13, thereby providing contour correction. It should be appreciated that the comb filter 15 and subtractor 16 comprise a luminance signal/color signal separation circuit.
With such a prior art vertical contour correcting circuit, only the color signal component passes through the vertical BPF 11, the signals A, B, and C have a time difference of 140 nsec, i.e., a phase difference of 180.degree., with each other, even though they are passed through the LPF 12; as such, for horizontal transition of color, a mixture of color phase information before and after such transition is outputted from the adder 126. Thus, if the luminance signal is contour-corrected with a correction signal containing such color phase information, improper contour correction results, so that dot interference might occur in color transition areas of a reproduced image.
Accordingly, it is an object of the present invention to provide a vertical contour correcting circuit that provides proper contour correction on a luminance signal in a horizontal color transition area of an image, thereby allowing for reduction of dot interference in the color transition area of a reproduced image.